This invention relates to a flow control solenoid valve apparatus and, more particularly, to a the flow control solenoid valve apparatus for electrically controlling flow rate of a working oil of a vehicular steering apparatus or other industrial apparatus.
FIG. 4 is a sectional view of a conventional solenoid valve of the type to which the present invention is directed, and FIG. 5 is a block diagram of the solenoid and the controller of the solenoid valve illustrated in FIG. 4.
In FIGS. 4 and 5, the solenoid valve apparatus comprises a valve body 1 having formed therein a central valve bore 2. The central bore 2 is provided with a first output port 3a, a second output port 3b and a supply port 3c between the first and the second output ports 3a and 3b and connectable to an oil pump 10. A drain port 3d is formed outside of the first and the second output ports so that it is connected to an oil tank 12 through a filter 11.
Within the central valve bore 2, a valve spool 4 is slidably disposed. The valve spool 4 is in an underlapping relationship with respect to the valve bore 2. The valve spool 4 is elastically supported at its opposite ends by centering springs 5a and 5b and biased to a neutral position illustrated in the figure. Between each of the centering springs 5a and 5b and the valve spool 4 are retainers 6a and 6b, which prevent either one of the springs 5a and 5b from acting on the valve spool 4 when it is shifted away from that spring 5a or 5b beyond a certain limit.
At one end of the valve body 1, a solenoid actuator 21 for actuating the valve spool 4 is mounted. The solenoid actuator 21 comprises a hollow cylindrical yoke 22 having secured at its opposite ends a first magnetic core 23 and a second magnetic core 24. Also, a permanent magnet 25 magnetized in the radial direction is disposed at an axially central position within the yoke 22, and a first coil 26 wound on a coil bobbin 28 is mounted between the permanent magnet 25 and the first magnetic core 23, and a second coil 27 wound on a coil bobbin 29 is mounted between the permanent magnet 25 and the second magnetic core 24. The first coil 26 and the second coil 27 are electrically connected in series as illustrated in FIG. 5.
Mounted on the inner surface of the permanent magnet 25 is a third core 30 within an inner surface of which a sleeve bearing 31 is press fitted. The sleeve bearing 31 has on its inner surface a tetrafluoroethylene (Teflon: Trade Name) coating layer of about 0.3 mm to provide a low friction coefficient and a cylindrical magnetic gap (sliding gap) in relation to a plunger 32 slidable within the sleeve bearing 31.
Each portion of the first and the second magnetic cores 23 and 24 opposing the end faces of the plunger 32 is provided with a cylindrical portion 33 or 34 having a diameter larger than that of the plunger 32 in order that the axial dimensions of the cylindrical magnetic gap change in response to the axial position of the plunger 32.
Reference numeral 41 indicates a controller for controlling a current supply to the solenoid actuator 21 by chopping a dc current from a dc source 42 and varying the duty cycle (ON-OFF ratio) of the supplied current.
The operation will now be described. When the first and second coils 26 and 27 of the solenoid actuator 21 are not energized, the magnetic flux .phi.c shown by dash line in FIG. 4 is not provided, so that only first and second magnetic fluxes .phi.m1 and .phi.m2 generated in opposite directions from the N pole of the permanent magnet 25 are maintained. The first magnetic flux .phi.m1 appears in a first closed magnetic circuit which extends from the N pole of the permanent magnet 25, through the yoke 22, the first core 23, the cylindrical portion 33, the plunger 32, the sleeve bearing 31, and the third core 30 to return into the S pole of the permanent magnet 25. The second magnetic flux .phi.m2 appears in a second closed magnetic circuit which extends from the N pole of the permanent magnet 25, through the yoke 22, the second core 24, the cylindrical portion 34, the plunger 32, the sleeve bearing 31 and the third core 30 to return into the S pole of the permanent magnet 25.
A leftward attraction force is generated by the magnetic flux .phi.m1 between the cylindrical portion 33 and the left end of the plunger 32, and a rightward attraction force is generated by the magnetic flux .phi.m2 between the cylindrical portion 34 and the right end of the plunger 32. Since both attraction forces are substantially equal and the valve spool 4 is biased in the central position by the action of the centering springs 5a and 5b, the valve spool 4 is maintained in the neutral position, so that the high pressure working oil introduced from the supply port 3c is returned to the oil tank 12 through throttles 7a and 7b at the supply sides, throttles 8a and 8b at the drain sides and through the drain port 3d.
At this time, since the throttles on the right and left sides of the valve are equal to each other, the fluid pressure at the output ports 3a and 3b is kept equal, so that the piston 14 of a power cylinder 13 is maintained at the illustrated position.
When the first and second coils 26 and 27 of the solenoid actuator 21 are energized, the magnetic flux .phi.c illustrated by dash line in FIG. 4 is generated to extend through the yoke 22, the first core 23, the cylindrical portion 33, the plunger 32, the cylindrical portion 34 and the second core 24.
At this time, the resultant magnetic flux which passes through the cylindrical portion 33 of the first core 23 and the left end of the plunger 32 is a sum of the first magnetic flux .phi.m1 generated by the permanent magnet 25 and the magnetic flux .phi.c generated by the first and second coils 26 and 27, so that the magnetic attractive force toward the left acting on the plunger 32 is increased.
On the other hand, the magnetic flux which passes through the cylindrical portion 34 of the first core 24 and the right end of the plunger 32 is a substraction between the second magnetic flux .phi.m2 generated by the permanent magnet 25 and the magnetic flux .phi.c generated by first and the second coils 26 and 27, so that the magnetic attractive force toward the right acting on the plunger 32 is decreased.
The plunger 32 is subjected to a leftward attractive force generated at its left end and a rightward attractive force generated at its right end which result in a leftward differential drive force, so that the plunger 32 is moved to the left as viewed in the figure to a position where the drive force equals the centering spring 5a force action against the valve spool 4.
Since the magnetic gap at the attraction portion is cylindrical, an attractive force of a substantially flat profile is generated when the plunger 32 is positioned on the left side of the neutral position.
When the valve spool 4 is moved to the left by the force generated by the solenoid actuator 21, the throttle 7a is widened whereas the throttle 7b is narrowed. Also, the throttle 8a on the drain side is narrowed and the throttle 8b on the drain side is widened.
This causes the oil pressure within the first output port 3a to increase and the oil pressure within the second output port 3b to decrease, so that the piston 14 of the power cylinder 13 is moved to the right.
When the direction of currents flowing through the first and the second coils 26 and 27 of the solenoid actuator 21 are changed so that a magnetic flux opposite to the magnetic flux .phi.c of FIG. 4 is generated, the operation is similar but opposite in direction, so that the description of the operation is omitted.
Since the conventional solenoid flow rate control apparatus is constructed as above described, the coils of the solenoid actuator 21 must be of a large-current type which generates a very large drive force so that the valve spool 4 can shear dust particles in the working oil caught within the valve and can be moved as designed. When large-current type solenoid coils are used, the current flowing through the solenoid during normal flow rate controlling is decreased, disadvantageously degrading the flow rate controllability of the fluid.